The present invention relates to grains of silver halide (hereinafter silver halide is also referred to as "AgX") which are useful in the field of photography and relates to a photographic lightsensitive material including the same. More particularly, the present invention relates to silver halide grains which are excellent in the ratio of sensitivity/granularity and relates to a photographic lightsensitive material including the same.
Highly sensitive color lightsensitive materials are required for photographing special scenes, such as those of sports needing a high shutter speed and those of stages where the amount of light cannot meet the requirement for exposure, in the field of color photographic lightsensitive materials, especially, color reversal lightsensitive materials often used by professional photographers. The highly sensitive color photographic lightsensitive materials, however, generally have poor graininess (coarse), so that an improvement in the sensitivity/graininess relationship is demanded.
Various methods are available for enhancing the sensitivity of the silver halide emulsion, which include, for example, method (1) in which the number of photons absorbed by each of the grains is increased, method (2) in which the efficiency of conversion of photoelectrons generated by light absorption into silver cluster (latent image) is enhanced and method (3) in which a development activity is enhanced for effective utilization of the formed latent image.
In line with the method (1) out of the above methods, tabular AgX grains having twin planes parallel to each other, having principal planes composed of {111} faces and having a high aspect ratio (projected circle equivalent diameter/thickness) have become frequently used in the photographic lightsensitive materials because, as compared with the nontabular AgX grains, the proportion of incident light which passes through the lightsensitive layer without being absorbed is reduced to thereby enable enhancing a light capturing efficiency and because improvement can be attained in image quality (covering power, sharpness and graininess), rate of development, spectral sensitization characteristics, etc.
Although the sensitivity/granularity ratio of employed emulsion must be further raised for further enhancing the image quality of photographic lightsensitive materials, the situation has been encountered such that, with the use of tabular grains having principal planes composed of {111} faces, the sensitivity increase is limited to thereby disenable attaining the object.
In this situation, the inventor has decided to study on tabular grains each having principal planes composed of {100} faces. Because, as compared with {111} faces, almost all of which are composed of halide ions (hereinafter a halide ion is also referred to as "X.sup.- "), grains having {100} faces, each of which composed of alternately arranged Ag.sup.+ and X.sup.-, are presumed to exhibit less desensitization in the case where a spectral sensitizing dye is adsorbed thereto and may promise enhanced photographic performance, and are thus attracting attention in recent years.
With respect to the preparation of these {100} tabular grains, the preparation of silver bromide {100} tabular grains is described in U.S. Pat. Nos. 4,063,951 and 4,386,156. However, the ultimate use of the grains prepared by the methods of these patents in photographic lightsensitive materials resulted in unsatisfactory performance, especially, in respect of the photographic sensitivity. In efforts to attain further performance enhancement, Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as JP-A-) 6-19028 discloses a process for preparing silver iodobromide {100} tabular grains which contain {100} tabular grains in an amount of at least 50% based on the total projected area of all grains and which exhibit an intragranular average silver iodide content of at least 1 mol %. This process has infallibly enabled obtaining {100} tabular grains which exhibit enhanced performance as compared with that of the grains described in the prior patents. However, the performance of the {100} tabular grains is still unsatisfactory for use in the high-image-quality photographic lightsensitive materials of recent years. In particular, the performance in small-size region of the {100} tabular grains is inferior to that of the conventional {111} tabular emulsion. For example, in the use in the color reversal photographic lightsensitive material whose performance is conspicuously affected by the emulsion performance in small-size region, the {100} tabular grains leave room for performance enhancement in attainments of high sensitivity and high contrast, etc.